A. Field of the Invention
The present invention relates to magnetic recording media installed in various types of magnetic recording devices.
B. Description of the Related Art
“Perpendicular magnetic recording systems” were put to practical application recently as a technology to achieve high recording density in magnetic recording. This system, in which recording magnetization is perpendicular to the plane of the recording medium, is taking the place of the conventional longitudinal magnetic recording system, in which the recording magnetization is parallel to the plane of the recording medium. A perpendicular magnetic recording medium (hereinafter abbreviated as a perpendicular medium) used in perpendicular magnetic recording is principally composed of a magnetic recording layer of a hard magnetic material, an underlayer for aligning the recording magnetization of the magnetic recording layer in the perpendicular direction, a protective layer for protecting the surface of the magnetic recording layer, and a backing layer of a soft magnetic material for concentrating a magnetic flux generated by the magnetic head that is used for recording on the recording layer.
It is one of guidelines in medium design for high recording density that magnetic separation between magnetic grains composing the magnetic recording layer should be enhanced in order to minimize a magnetization reversal unit. Since the thickness of the magnetic recording layer is basically uniform in the direction over the medium surface, reduction of the magnetization reversal unit means a decrease in a cross sectional area of the magnetization reversal unit while maintaining the height. As a result, a demagnetizing field acting thereon decreases while increasing a reversing magnetic field. Thus, regarding the configuration of the magnetization reversal unit, enhancement of recording density needs a larger write magnetic field.
On the other hand, it is known that an energy value Ku V of a grain needs to be sufficiently large relative to thermal energy kT in order to secure long term stability of recorded signals. Here, k is the Boltzmann constant; T, an absolute temperature; Ku, a crystalline magnetic anisotropy constant; and V, an activation volume. A decrease in size of the magnetization reversal unit as mentioned above means a decrease in V, which affects signal instability, creating a problem of so-called “thermal fluctuation.” In order to avoid this phenomenon, the Ku value must be increased, which also brings an increase in write magnetic field since the Ku value is in proportional relationship with the reversing magnetic field.
To cope with this problem, a technique has been proposed in which a structure includes two magnetic layers, and an exchange coupling energy between the layers is reduced to decrease the reversing magnetic field without deteriorating thermal stability. This type of medium is called an exchange coupling controlled medium. Japanese Unexamined Patent Application Publication No. 2005-310368, for example, discloses that the reversing magnetic field decreases with weakening of the exchange coupling energy from a condition of direct lamination of two magnetic layers at which the exchange coupling energy is infinitely large, takes the minimum value at the optimum coupling energy, and increases with a decrease towards zero in the coupling energy. This phenomenon is caused by non-simultaneous magnetization reversal processes, namely incoherent magnetization reversal processes, of the two magnetic layers while maintaining weak coupling between the two layers. Consequently, the optimum value of the coupling energy and consequently, a reduction rate of the reversing magnetic field, varies depending on physical properties such as saturation magnetization Ms and the Ku value of the upper and lower magnetic layers. For practical purposes, a coupling energy control layer is provided to change the coupling energy and optimize the physical properties of the upper and lower magnetic layers.
Another approach to the problem of write performance has been proposed which is a recording method called thermally assisted recording, in which a combination with a magnetic head is taken into consideration. This method utilizes a characteristic of magnetic materials, a temperature dependence of the Ku, which decreases with increase in temperature. A write process in this method is conducted during temporary decrease in the Ku value attained by heating the magnetic recording layer to reduce the reversing magnetic field. After the temperature has returned or decreased, the Ku value restores the original high value, so the recording signals are safely retained. When this new recording method is envisaged, design of a magnetic recording layer must consider temperature characteristics of the layer in addition to the conventional guideline. Technical Report of IECE, MR2004-39 (2004) discloses that the transition width of a recording bit is determined by a head magnetic field gradient and a temperature gradient. Since a large temperature change results in an increased difference in the Ku value, the difference between a write stage and a retaining stage increases, giving a large resulting gain. Regarding this item, it is known that the reversing magnetic field varies linearly with temperature variation in CoPt alloy magnetic materials, which are mainly used in perpendicular media at present and classified into ferromagnetic materials. These magnetic materials are also known to exhibit relatively small variation in the temperature gradient due to the composition; giving values generally smaller than −20 Oe/° C. On the other hand, magnetic materials such as TbFeCo, for example, which are commonly used in magneto-optical recording media and classified into ferrimagnetic recording materials, exhibit large composition dependence of the temperature gradient of reversing magnetic field larger than −100 Oe/° C. by setting a compensation temperature at around the recording temperature. In addition to the material selection, there is a method for controlling overall temperature variation, which employs a lamination structure of plural layers of the ferromagnetic and ferrimagnetic materials and in particular, uses a ferrimagnetic or an equivalent material as a switching layer, to generate or eliminate the exchange coupling energy at a recording temperature. For, example, JP 2005-310368 proposes a variety of layer structures.
The exchange coupling controlled media use an existing CoPt alloy magnetic material and control the balance between the reversing magnetic field and the thermal stability by devising a combination of the compositions of the layers. There is a limit, however, due to restriction of material properties including the Ms and the Ku values.
In thermally assisted recording, on the other hand, regarding an especially important issue of temperature variation control, conventional simple use of a CoPt alloy magnetic material hardly attains a temperature gradient of the reversing magnetic field over −20 Oe/° C. Use of a ferrimagnetic material, however, allows easy temperature control. When a transition metal/rare earth amorphous alloy material is used, however, the magnetization mechanism is so-called a type of magnetic domain wall movement and hard to fix magnetic domain walls, which are boundaries between bits. Fluctuation of the bit boundaries is undesirable from the view point of high density recording. Despite the need for narrowing widths of the magnetic domain walls in order to enhance recording density, it is difficult to reduce the width to a grain boundary width (about 1 nm), which is believed to be a boundary between bits in a micro grain system. Further, it is difficult to add a non-magnetic substance and to form a microscopic structure with magnetic grains surrounded by the nonmagnetic substance, as seen in the CoPt alloy magnetic materials.
The studies by the inventor of the present invention have clarified that, in combination of the exchange coupling control medium with the thermally assisted recording, media with more weakened exchange coupling energy exhibit a smaller temperature gradient. This fact means that simple application of an exchange coupling medium to the thermally assisted recording results in very little difference in the reversing magnetic field at a heated state from the one at an unheated state, thus, the primary object of the thermally assisted recording method cannot be achieved. This has been attributed to little variation of the exchange coupling energy itself with variation of temperature, that is, to variation of the optimum value of the coupling energy depending on the material properties of the upper and lower magnetic layers that have been changed at the heated state.
As described above, conventional recording systems possibly achieve a breakthrough by means of thermally assisted recording to attain high recording density in magnetic recording devices. Nevertheless, such a medium applicable to this recording system is yet needed that provides compatibility between high density write process and temperature characteristics control.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.